Studies of peptide coexistence have revealed two CRF neuron sub-types in the hypothalamus (PVN), CRF/AVP+ and CRF+/AVP- with respect to coexistence of these two peptides. Each subtype represents about half of the total CRF+ neuronal population in the PVN and of the total CRF+ neuronal population in the PVN and of the total CRF+ axonal population in the median eminence. The existence of these subtypes can account for the variations in CRF to AVP ratios in portal blood with different forms of stress. Analysis of the development of vasopressin and oxytocin neurons by immunocytochemical and RIA procedures have shown that there is a profound differential expression of peptides during fetal development. The vasopressin (AVP) neurons are more precocious than oxytocin (OT) neurons during development in extent of peptide expression, and in efficiency and extent of precursor processing, so that a stable intermediate form of oxytocin (OT- Gly10-Lys11-Arg12) is found during embryonic life. Vasotocin (AVT) and mesotocin (MT) neurons have been mapped in 3- dimensions in adult Xenopus hypothalamus, and neurophysin immunoreactivities have been documented. Expression of these peptides appears at least at stage 45 in Xenopus development. An organotypic tissue culture model of rat LHRH neurons has been developed, and preliminary data suggests trophic interactions between brain stem, hypothalamus, and pituitary regions in vitro studies on the opioid peptide sensitivity of AVP and OT secretion from the neural lobe in vitro have shown that OT secretion is inhibited by an endogenous opioid which appears to be secreted from the VP terminals, and also by exogenous administered dynorphin. *This project has been transferred from the LNN, DIR, NICHD (formerly ZO1-HD-00058-11 LNN) to the LNC, DIR, NINCDS.